The invention relates to a method for applying a multilayer wear-resistant coating on metallic, optionally already coated, surfaces, wherein the coating is composed of at least two anti-wear layers and an intermediate layer each arranged between two anti-wear layers, as well as a coating for a metallic surface.
With heavily loaded component parts as they are, for instance, encountered in the servo-valve or nozzle of an injector in a common-rail injection system, it has been common in the prior art to realize a coating by using especially hard materials in order to increase both hardness and wear-resistance. A coating comprising several layers also belongs to the prior art.
In doing so, it is desirable, in order to achieve the required wear indices, to be able to apply layer thicknesses as large as possible, wherein, however, the use of especially hard materials as coating materials will limit the layer thickness on account of the residual stresses occurring within the layer and rising with the layer thickness. Residual stresses in the layer will, as a rule, lead to the formation of cracks in the coating and/or chipping off. In this context, a layer sequence has been proposed in WO 2006/005288 A1, in which an adhesive layer based on Cr is initially applied on a metallic surface, a CrN gradient layer is applied on the adhesive layer, and at least one covering layer having a constant composition based on CrN, Cr2N or a mixture of the two phases is applied on the CrN gradient layer. The CrN layer is characterized by relatively low residual stresses so as to enable the application of CrN layers in clearly larger layer thicknesses.
From DE 102004002678 B4, a multilayer coating for the valve needle of a valve is to be taken, wherein the layer sequence comprises at least a first adhesive layer, a first anti-wear layer, a second adhesive layer and a second anti-wear layer, said layer sequence being repeatedly applicable several times, if required. With such a configuration, a plurality of anti-wear layers of small layer-thicknesses can be assembled to an altogether sufficiently thick coating. The individual anti-wear layers can be applied in thicknesses, which ensure that no excessive residual stresses occur, eventually causing the anti-wear layers to chip off. The adhesive layers applied between the anti-wear layers in that case are made of materials markedly softer than the material of the anti-wear layer.
According to the prior art, it is thus provided that individual anti-wear layers made of highly wear-resistant, hard materials are combined with adjoining or interposed intermediate, transition or adhesive layers made of materials softer than the material of the anti-wear layer so as to increase the overall coating thickness, wherein the necessary elasticity to prevent the harder material from chipping off is provided by the softer material arranged therebetween. The increase in the overall coating thickness is, however, opposed by the local reduction of the wear resistance caused by the softer material of the intermediate layers. Under extreme loads, a removal of the uppermost applied anti-wear layer will unavoidably be caused, thus exposing the underlying intermediate layer made of a less wear-resistant material, which, in the following, will be relatively rapidly worn such that a layer structure of this type will, in the main, be prone to relatively rapid wear.
The present invention, therefore, aims to increase the wear resistance of a multilayer coating in which an intermediate layer is each arranged between individual anti-wear layers, and to provide a method for producing such a multilayer coating.
To solve this object, the invention is essentially characterized in that the intermediate layer is comprised of a material composition containing the material of the anti-wear layer and a further material, wherein the application of the intermediate layer is effected with a content of the material of the anti-wear layer decreasing over a first transition region and a content of the material of the anti-wear layer increasing over a second transition region, the content of the material of the anti-wear layer in the intermediate layer being selected to be at least 5% by weight at every point. In such a layer structure, transition regions in which the content of the material of the adjacent anti-wear layers increases or decreases, respectively, are formed in the respective intermediate layer, with a material different from the material of the anti-wear layer being admixed at a respectively increasing or decreasing content. The overall content of the further material in the intermediate layer is, however, limited such that, according to the invention, a minimum content of 5% by weight of the material of the anti-wear layer is contained in the intermediate layer at every point. It is thereby safeguarded that the wear properties will be maintained in the intermediate layer while, at the same time, kind of a relaxation zone will nevertheless be produced between two anti-wear layers to prevent chipping or cracking caused by residual stresses occurring in the material of these layers.
In order to achieve as continuous a transition as possible between the individual layers, it is preferably provided that the content of the material of the anti-wear layer in the intermediate layer is decreased or increased according to a ramp function. In order to further ensure that the intermediate layer will exhibit sufficient wear resistance even under high loads, it is preferably provided that the content of the material of the anti-wear layer in the intermediate layer is decreased to at least 30% by weight, preferably to at least 50% by weight. This will preferably result in a layer structure in which an anti-wear layer, i.e. a layer having a content of 100% by weight of coating material, is followed by a first transition region of the intermediate layer, in which the content of the material of the anti-wear layer is decreased from 100% by weight to, for instance, 50% by weight and the content of the further material is increased accordingly. For the transition to the next superimposed anti-wear layer of pure material, a further transition region of the intermediate layer is subsequently provided, in which the content of the material of the anti-wear layer is again increased to 100% by weight and the content of the further material is decreased accordingly. The first transition region preferably directly adjoins the anti-wear layer located therebelow, and the second transition region directly merges into the anti-wear layer arranged thereabove. Between the first transition region, in which the content of the material of the anti-wear layer is lowered, and the second transition region, in which the content of the material of the anti-wear layer is again raised, a further region in which the content of the materials is selected to remain unchanged may be arranged, if desired. A configuration in which the second transition region directly adjoins the first transition region is, however, preferred, because the intermediate layer is to be formed as thin as possible and no other functional subregions appear necessary besides the two transition regions serving the material transition.
In a preferred manner, the layers are applied by a CVD process, wherein processes are known from the prior art, in which the mass ratios of the individual materials can be continuously varied during the application of the layers.
In order to achieve a configuration as wear-resistant as possible, it is preferably provided that the anti-wear layer is comprised of a diamond-like carbon material (DLC). In addition to the material of the anti-wear layer, the intermediate layer preferably contains a further wear-resistant material such as, e.g. CrN.
In the layer sequence according to FIG. 1, an adhesive layer 2 of a soft material, e.g. Cr, is at first applied on the material 1 to be coated. After this, a first anti-wear layer 3 which may, for instance, be comprised of CrN is applied. In a transition region 4, the concentration of the material of the first anti-wear layer is decreased to zero and, at the same time, the concentration of the material of a second anti-wear layer 5 made, for instance, of DLC (diamond-like carbon) is increased. Subsequently, a second anti-wear layer 5 is applied. The diagram depicts the concentration 6 as a function of the distance 7 from the surface of the component part 1 to be coated, the full line 8 indicating the concentration of the material of the first anti-wear layer and the dot-and-dash line 9 representing the concentration of the material of the second anti-wear layer.
FIG. 2 illustrates a coating according to the present invention. Upon the already known layer sequence comprising the material 1 to be coated, the adhesive layer 2, a first anti-wear layer 3, a protective transition layer 4, and a second anti-wear layer 5 follows an intermediate layer 10 in which the concentration of the material of the adjacent anti-wear layers is decreased from 100% to 50% in a first transition region a and is again increased to 100% in a second transition region b, wherein a further material, i.e. the material of layer 3 in the present case, is admixed accordingly, with a content of the further material being raised up to 50% in the transition region a and again lowered to zero in the second transition region b following thereupon. This intermediate layer 10 is followed by a further ply of the second anti-wear layer 5. The application of the intermediate layer 10 and the second anti-wear layer 5 may subsequently be repeated several times.
The advantage of the present invention resides in that, due to the coating, the stresses occurring in the anti-wear layers 3 and 5 from a certain layer thickness will normally increase to such an extent that the coating will no longer safely adhere. The thickness of a coating is, thus, limited upwards. By introducing the special intermediate layer 10, stresses will be reduced, and a further ply of the same protective layer can be applied such that the realization of almost any coating thickness will be feasible in practice.
For the material of the anti-wear layer 5, DLC (diamond-like carbon) may be selected. For the material of the anti-wear layer 3, which is also admixed to the intermediate layer 10, CrN may, for instance, be selected. However, the material admixed to the intermediate layer 10 need not necessarily correspond with the material of the anti-wear layer 3.
It should further be noted that the layer structure formed by the repeated application of layers 5 and 10 need not necessarily be applied on the component part 1 via the interposition of layers 3, 4 and, optionally, 2. On the contrary, the direct application, with or without interposition of an adhesive layer, is also conceivable. The arrangement of layer 3, which is made of a softer material as compared to the material of the anti-wear layer 5, is merely effected to provide a continuous transition from the soft material of the component part 1 to the very hard material of the anti-wear layers 5.